Vitamins and nutrition . Patient guide
Vitamin D (25-OH Vitamin D) Blood Test
Also known as: 25-hydroxyvitamin D, Calcidiol
What is 25-OH Vitamin D
Vitamin D is a fat-soluble vitamin made in skin from sunlight and obtained in smaller amounts from food, with status measured by serum 25-hydroxyvitamin D (25-OH vitamin D). According to NICE CKS Vitamin D deficiency in adults, a serum 25-OH vitamin D below 25 nmol/L is deficient, 25 to 50 nmol/L is insufficient, and above 50 nmol/L is sufficient. Public Health England (now UKHSA) estimates that around 1 in 5 UK adults have low vitamin D status in winter, and SACN recommends 10 micrograms (400 IU) daily for the general UK population aged 4 and over during autumn and winter.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in nmol/L. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Deficient (NICE CKS) | under 25 | nmol/L |
| Insufficient | 25 to 50 | |
| Sufficient (NICE / SACN) | over 50 | |
| Often targeted clinically for symptomatic patients | 75 to 125 | expert opinion, not a NICE threshold |
| Above which supplementation should be reviewed | over 220 |
What it is
Skin makes vitamin D3 (cholecalciferol) when exposed to ultraviolet B sunlight. The liver converts it to 25-hydroxyvitamin D, the main circulating form and the best measure of vitamin D status in blood tests. The kidneys then convert it to its active hormonal form, 1,25-dihydroxyvitamin D, when calcium balance demands it.
Why a clinician would order it
Vitamin D status is checked in fatigue, low mood, hair thinning, bone or muscle ache, frequent infections, in patients with darker skin (where UK sunlight is rarely enough), and as part of preventative wellness screening. UK guidance recommends most adults supplement in winter.
If your level is outside the range
Symptoms of low 25-OH Vitamin D
- Persistent fatigue
- Low mood
- Bone or muscle ache
- Hair thinning
- Frequent infections
- Reduced exercise tolerance
What low can indicate. Insufficient sunlight exposure (the commonest UK cause), darker skin, obesity (vitamin D is fat-soluble and less available), malabsorption (coeliac or Crohn's disease, weight loss surgery), or older age (reduced skin synthesis).
Symptoms of high 25-OH Vitamin D
- Nausea, vomiting (excessive supplementation only)
- Confusion, kidney impairment (severe excess only)
What high can indicate. Excessive supplementation. Rarely, granulomatous disease such as sarcoidosis.
Testing tips
No fasting required. If you take a vitamin D supplement, the result will reflect supplemented status (useful for adjusting dose). UK levels are typically highest around September and lowest around February and March (SACN 2016).
If you book Vitamin D as part of Hair Loss Essentials, Advanced Hair & Hormone Check, Pre-Transplant Screening, Pre-Transplant + BBV Screen or General Wellness, the panel needs a fast of at least 12 hours (water is fine), so follow the panel instructions rather than the single-marker advice above.
Related reading
- Vitamin D Deficiency: The Symptoms, and the Blood Test That Catches It Vitamin D deficiency is common in the UK and easy to miss. The symptoms, who is at risk, what the blood test checks, and the level worth aiming for.
- Should I Get My Vitamin D Checked This Winter? What UK Sunlight Does to Your Levels From October to early March, UK sunlight is too weak for your skin to make enough vitamin D. Who is most at risk, when to test, and what results mean.
- Is There a SAD Blood Test? What to Rule Out There is no blood test for SAD: a doctor diagnoses it. But thyroid, vitamin D, iron and B12 problems can look alike. What a test can rule out.
Where you can get this tested
Vitamin D is included in the following WMG Health panels. Same-day appointments are usually available at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Looking to book a test?
Comprehensive private health screening
Full blood screening at 134 Harley Street, London, reviewed by GMC-registered doctors with a written interpretation.
Symptoms often investigated with Vitamin D
Vitamin D is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.
Common questions about 25-OH Vitamin D
What is a normal 25-OH Vitamin D range?
Deficient (NICE CKS): under 25 (nmol/L). Insufficient: 25 to 50. Sufficient (NICE / SACN): over 50. Often targeted clinically for symptomatic patients: 75 to 125 (expert opinion, not a NICE threshold). Above which supplementation should be reviewed: over 220. Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low 25-OH Vitamin D result mean?
Insufficient sunlight exposure (the commonest UK cause), darker skin, obesity (vitamin D is fat-soluble and less available), malabsorption (coeliac or Crohn's disease, weight loss surgery), or older age (reduced skin synthesis).
What does a high 25-OH Vitamin D result mean?
Excessive supplementation. Rarely, granulomatous disease such as sarcoidosis.
Do I need to fast or prepare for the 25-OH Vitamin D blood test?
No fasting required. If you take a vitamin D supplement, the result will reflect supplemented status (useful for adjusting dose). UK levels are typically highest around September and lowest around February and March (SACN 2016). If you book Vitamin D as part of Hair Loss Essentials, Advanced Hair & Hormone Check, Pre-Transplant Screening, Pre-Transplant + BBV Screen or General Wellness, the panel needs a fast of at least 12 hours (water is fine), so follow the panel instructions rather than the single-marker advice above.
What is a vitamin D blood test called on a lab report?
It is usually reported as 25-hydroxyvitamin D, 25-OH vitamin D or 25(OH)D, and sometimes as calcidiol. It is the standard test of vitamin D status. A separate vitamin D3 (cholecalciferol) test exists but is a more specialised request, and most people do not need it: see our vitamin D3 page for how the two differ.
What vitamin D level is low in the UK?
NICE CKS treats a 25-OH vitamin D below 25 nmol/L as deficient, 25 to 50 nmol/L as insufficient and above 50 nmol/L as sufficient. A clinician reads your number with your symptoms, season and supplements, and the result is not a diagnosis on its own.
Can I order a 25-OH Vitamin D blood test privately in London?
Yes. WMG Health offers 25-OH Vitamin D in several standard panels and as part of a custom panel at our 134 Harley Street clinic. Results are released as each marker is reviewed, and the completion date is set by the slowest marker in your panel. We confirm the expected timeframe at booking. Book a standard panel online, or ask for a custom panel via /contact/ or 020 3239 3378.